全文获取类型
收费全文 | 8985篇 |
免费 | 1712篇 |
国内免费 | 967篇 |
专业分类
化学 | 6056篇 |
晶体学 | 86篇 |
力学 | 730篇 |
综合类 | 65篇 |
数学 | 1132篇 |
物理学 | 3595篇 |
出版年
2024年 | 38篇 |
2023年 | 189篇 |
2022年 | 308篇 |
2021年 | 331篇 |
2020年 | 353篇 |
2019年 | 324篇 |
2018年 | 296篇 |
2017年 | 296篇 |
2016年 | 435篇 |
2015年 | 426篇 |
2014年 | 473篇 |
2013年 | 653篇 |
2012年 | 763篇 |
2011年 | 845篇 |
2010年 | 562篇 |
2009年 | 535篇 |
2008年 | 634篇 |
2007年 | 490篇 |
2006年 | 523篇 |
2005年 | 391篇 |
2004年 | 327篇 |
2003年 | 259篇 |
2002年 | 237篇 |
2001年 | 202篇 |
2000年 | 193篇 |
1999年 | 194篇 |
1998年 | 182篇 |
1997年 | 173篇 |
1996年 | 169篇 |
1995年 | 137篇 |
1994年 | 121篇 |
1993年 | 112篇 |
1992年 | 91篇 |
1991年 | 69篇 |
1990年 | 75篇 |
1989年 | 61篇 |
1988年 | 45篇 |
1987年 | 34篇 |
1986年 | 34篇 |
1985年 | 22篇 |
1984年 | 18篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 3篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1975年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
A multiscale information measure (MIM), calculable from per-pixel wavelet coefficients, but relying on global statistics of synthetic aperture radar (SAR) image, is proposed. It fully exploits the variations in speckle pattern when the image resolution varies from course to fine, thus it can capture the intrinsic texture of the scene backscatter and the texture due to speckle simultaneously. Graph spectral segmen- tation methods based on MIM and the usual similarity measure are carried out on two real SAR images. Experimental results show that MIM can characterize texture information of SAR image more effectively than the commonly used similarity measure. 相似文献
982.
本文研究了不同衬底温度对Ga液滴在Al_(0.4)Ga_(0.6)As表面形成纳米结构的影响,当300℃≤T≤380℃时,Ga液滴演化成纳米孔(Nanohole)和盘状结构(diffusion halo),纳米结构的尺寸随温度升高而增大.当T≥385℃时,盘状结构消失,形成一定平坦的Al_xGa_(1-x)As薄膜,Ga液滴在界面处继续向下刻蚀直至耗尽,形成平均直径为75 nm,平均孔深为5.52 nm的纳米孔.本文还通过盘状结构测出平均扩散长度△R,并拟合出Ga原子在Al_(0.4)Ga_(0.6)As表面的激活能E_A=0.78(±0.01) eV和扩散前因子D_0=0.15(×4.1~(±1))10~(-2 ) cm~2s~(-1). 相似文献
983.
Charm quark energy loss in QCD matter 总被引:1,自引:0,他引:1
W.?C.?XiangEmail author H.?T.?Ding D.?C.?Zhou D.?Rohrich 《The European Physical Journal A - Hadrons and Nuclei》2005,25(1):75-78
The energy loss of heavy quarks in a quark-gluon plasma of finite size is studied within the light-cone path integral approach. A simple analytical formulation of the radiative energy loss of heavy quarks is derived. This provides a convenient way to quantitatively estimate the quark energy loss. Our results show that if the energy of a heavy quark is much larger than its mass, the radiative energy loss approaches the radiative energy loss of light quarks. 相似文献
984.
Summary An exact, three-dimensional analysis is developed for a penny-shaped crack in an infinite transversely isotropic piezoelectric
medium. The crack is assumed to be parallel to the plane of isotropy, with its faces subjected to a couple of concentrated
normal forces and a couple of point electric charges that are antisymmetric with respect to the crack plane. The fundamental
solution of a concentrated force and a point charge acting on the surface of a piezoelectric half-space is employed to derive
the integral equations for the general boundary value problem. For the above antisymmetric crack problem, complete expressions
for the elastoelectric field are obtained. A numerical calculation is finally performed to show the piezoelectric effect in
piezoelectric materials. It is noted here that the present analysis is an extension of Fabrikant's theory for elasticity.
Received 30 August 1999; accepted for publication 1 March 2000 相似文献
985.
在fullerene-PBTDP-TPA三体体系中的光诱导的分子内电荷转移的实验基础上,分别利用密度泛函理论和含时密度泛函理论以及结合二维、三维实空分析方法对fullerene、PBTDP-TPA二体以及fullerene-PBTDP-TPA三体的基态和激发态的性质进行了理论研究.二维定点表象揭示了激发情况下电子和空穴的相干情况.三维跃迁矩阵展示了跃迁偶极矩的方向和强度,而且三维电荷差密度给出了光诱导的分子内电荷转移的方向和结果.同时,用二维和三维表象的方法对fullerene-PBTDP-TPA三体中光诱导的分子内电荷转移过程进行了验证,揭示了在给予者-桥-接收者三体分子体系中分子内激发时电荷转移的机制.除此之外,还发现直接从给予者到接收者超交换的分子内电荷转移极大地促进了给予者-桥-接收者三体分子体系内的电荷转移. 相似文献
986.
987.
Derong Ding 《Tetrahedron letters》2010,51(9):1322-6795
2-Amino-8-oxo-tetrahydro-4H-chromene-3-carbonitriles were synthesized for the first time from a tandem Michael addition-cyclization reaction between cyclohexane-1,2-dione and benzylidenemalononitriles. An enantioselective synthesis of these compounds was achieved in moderate ee values (up to 63% ee) by using a cinchona alkaloid-derived thiourea catalyst. 相似文献
988.
J. H. Liu L. P. Wang Q. M. Xiao Z. Yao H. Ding 《Plasma Chemistry and Plasma Processing》2010,30(3):349-361
This work reports laser ionization combined with Time-Of-Flight (TOF) mass spectrometry investigation on intermediate species
in the hydrocarbon plasma of atmospheric-pressure fast-flow pulsed dc-discharge. All neutral intermediate species including
transient radicals from benzene/Ar discharge have been characterized by a molecular beam sampling combined with TOF mass spectrometry.
This shows that with a hydrocarbon gas mixture of 0.3% C6H6 in Ar discharge the intermediate species consist of simple radicals (such as C2, C5H5, C7H7) and polycyclic organic molecules (C10H8, C13H10, C14H10). Theoretical studies on total energies and ionization potentials of the intermediate species have been carried out using
the hybrid density functional theory. Effect of the ionization potential on mass spectral intensity has been discussed. Based
on the observed data, the possible major neutral reaction channels of the plasma chemistry have been discussed. The developed
experimental method has implications in volatile organic compounds removing and impurities diagnosis in Tokamak edge-plasma. 相似文献
989.
990.
A multicompartment electrolyzer, which has been used for preparative electrophoresis [Z. Liu, Z. Huang, J.-Y. Cong, et al., Sep. Sci. Technol. 31 (1996) 427], is applied for carrying out affinity chromatography in an alternating electric field. The effect of electric field strength on the adsorption and desorption characteristics is experimentally examined with human serum albumin and Blue Sepharose Fast Flow as a model system. It is shown that the existence of an electric field leads to a significant change in the adsorption capacity of the blue dye, which may be used for establishing a preferential adsorption to achieve a high resolution. The adsorption speed increases slightly with respect to the increase of electric field strength, while adsorption capacity in the presence of an electric field is independent of the electric field strength. Different elution behavior is observed as function of adsorption condition and a high recovery of the adsorbed protein is obtained when the adsorption is carried out in the presence of an electric field. 相似文献